Small molecules involved in molecular interactions with a therapeutic target, be it enzyme or receptor, are often described in terms of binding elements or pharmacophoric groups which directly interact with the target, and non-binding components which form the framework of the bioactive molecule. In the case of peptide ligands or substrates for instance, usually a number of amino acid side chains form direct interactions with their receptor or enzyme, whereas specific folds of the peptide backbone (and other amino acid residues) provide the structure or scaffold that controls the relative positioning of these side chains. In other words, the three dimensional structure of the peptide serves to present specific side chains in the required fashion suitable for binding to a therapeutic target. The problem is that such models do not allow for rapid identification of drug candidates owing to the necessity to synthesize an enormous amount of compounds to identify potential active compounds.
A pharmacophoric group in the context of these libraries is an appended group or substituent, or part thereof, which imparts pharmacological activity to the molecule.
Molecular diversity could be considered as consisting of diversity in pharmacophoric group combinations (diversity in substituents) and diversity in the way these pharmacophoric groups are presented (diversity in shape). Libraries of compounds in which either diversity of substituents, or diversity of shape, or both of these parameters are varied systematically are said to scan molecular diversity.
Carbohydrate scaffolds provide a unique opportunity to create libraries of structurally diverse molecules, by varying the pharmacophoric groups, the scaffold and the positions of attachment of the pharmacophoric groups in a systematic manner. Such diversity libraries allow the rapid identification of minimal components or fragments containing at least two pharmacophoric groups required for an interaction with a biological target. These fragments can be further optimized to provide potent molecules for drug design. Therefore these types of carbohydrate libraries provide an excellent basis for scanning molecular diversity.
In previous applications (WO2004014929 and WO2003082846) we demonstrated that arrays of novel compounds could be synthesized in a combinatorial manner. The libraries of molecules described in these inventions were synthesized in a manner such that the position, orientation and chemical characteristics of pharmacophoric groups around a range of chemical scaffolds, could be modified and/or controlled. These applications demonstrate the synthesis and biological activity of a number of new chemical entities.
Many drug discovery strategies fail owing to lack of knowledge of the bioactive conformation of, or the inability to successfully mimic the bioactive conformation of the natural ligand for a receptor. Libraries of compounds of the present invention allow for the systematic “scanning” of conformational space to identify the bioactive conformation of the target.
Typically in the prior art, libraries based on molecular diversity are generated in a random rather than a systematic manner. This type of random approach requires large number of compounds to be included in the library to scan for molecular diversity. Further, this approach may also result in gaps in the model because of not effectively accessing all available molecular space.
Therefore, one of the problems in the prior art is the necessity to synthesize an enormous amount of compounds to identify potential active compounds. Attempts have been made to develop peptidomimetics using sugar scaffolds by Sofia et al. (Bioorganic & Medicinal Chemistry Letters (2003) 13, 2185-2189). Sofia describes the synthesis of monosaccharide scaffolds, specifically containing a carboxylic acid group, a masked amino group (N3) and a hydroxyl group as substitution points on the scaffold, with the two remaining hydroxyl groups being converted to their methyl ethers. Sofia teaches a specific subset of scaffolds not encompassed by the present invention and does not contemplate methods to simplify the optimization of pharmacophoric groups.
Therefore there remains a need to provide a method of effectively scanning libraries designed from compounds with a wider range of different pharmacophoric groups.
The present invention is directed to a method of drug design utilizing iterative scanning libraries, resulting in surprisingly efficient identification of drug candidates, starting from a selected number of pharmacophores (e.g., two) in the first library and designing subsequent libraries with additional pharmacophores based on SAR information from the first library.
The invention can provide a new method for the rapid identification of active molecules.
In an embodiment, and to demonstrate the versatility of our invention, one of the G-protein coupled receptors (GPCR's) was chosen as a target: the somatostatin receptor (SST receptor). The tetradecapeptide somatostatin is widely distributed in the endocrine and exocrine system, where it has an essential role in regulating hormone secretion [1-3]. Five different subtypes have been identified to date (SST1-5), which are expressed in varying ratios throughout different tissues in the body. Somatostatin receptors are also expressed in tumours and peptide analogues of somatostatin affecting mainly SST5, such as ocreotide, lanreotide, vapreotide and seglitide [4-7] have antiproliferative effects. They are used clinically for the treatment of hormone-producing pituitary, pancreatic, and intestinal tumours. SST5 is also implicated in angiogenesis, opening up the possibility of developing antiangiogenic drugs that act on the SST5 receptor, for example for the use in oncology. The “core sequence” in somatostatin responsible for its biological activity is Ph-Trp-Lys (FWK), representing a motif of two aromatic groups and a positive charge, which is found in almost all SST receptor active compounds.
It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.